Activation volume and incipient plastic deformation of uniaxially-loaded gold nanowires at very high strain rates

Uniaxial tensile loading is investigated by the molecular dynamic (MD) method on Au nanowires at ultra-high strain rates. The activation volume is used to comprehensively characterize the incipient plastic deformation during this process. For lower strain rates such as 6.287 × 10⁸ s ^-1, the moving velocity, V, of the atom planes due to uniaxial loading is two orders of magnitude smaller than the phonon wave propagation speed, V₀, and the coherence between atoms is always maintained with a larger activation volume. In this case, plastic deformation is initiated mainly by collective atomic slipping, and thus only lower flow stress is needed. On the other hand, for higher strain rates such as 6.287 × 10 ¹⁰ s^-1, V is elevated to the same magnitude as V ₀, the atom coherence is broken, their individual behavior is dominated by extraordinarily small activation volume, and atom diffusion becomes the main mechanism for plastic deformation. As a result, the yield strength is improved substantially. A higher temperature may weaken this strain-rate-dependent mechanical behavior because of the enhanced atom activity.